This is an open label, phase I/II, multi-center study in adult participants with metastatic hormone sensitive prostate cancer (mHSPC) and with metastatic castration resistant prostate cancer (mCRPC) who have received prior anti-cancer treatment and have a positive 68Ga-PSMA-11 PET scan. The purpose of this study is to learn if the study drug, \[225Ac\]Ac-PSMA-R2, is safe and tolerable, and has anti-tumor activity in treated patients. Enrollment of all participants was not completed. The evaluated radioligand showed acceptable safety but limited benefit in metastatic prostate cancer